The present invention relates to a digital radio communication apparatus and, more particularly, to a communication apparatus such as a digital automobile telephone using a quadrature modulation scheme.
For example, a communication apparatus having the arrangement shown in FIG. 2 is used as a conventional communication apparatus. This arrangement is designed for analog mobile communication. An input signal to a transmission input terminal 31 is modulated by a modulator 27 using a direct modulation scheme. The modulated signal is output from a transmission output terminal 29. The frequency, of a signal received at a reception input terminal 28, corresponding to the frequency interval between transmission and reception frequencies is frequency-converted by a first frequency converter 21 using part of the output from the modulator 27. The resultant signal is used as a first IF signal for reception. The first IF signal is then amplified by a first IF amplifier 22 and is converted into a second IF frequency by a second IF converter 23 using a local oscillation signal from a local oscillator 26. The second IF signal is amplified by a second IF amplifier 24 and is demodulated by a demodulator 25 to obtain an output signal.
In this arrangement, since part of the output from the modulator 27 is used to obtain the first IF signal, an oscillator for generating a local oscillation signal for obtaining the first IF signal is not required.
If a communication apparatus having such an arrangement is applied to a digital radio communication apparatus, the apparatus requires a device capable of directly performing quadrature modulation by using a transmission frequency. An automobile telephone terminal unit requires an inexpensive device which can be operated with low current consumption and demands no adjustment. However, a quadrature modulator which can perform direct modulation and satisfy these requirements is not currently available. Under the circumstances, such a communication apparatus requires a means for performing quadrature modulation at a low frequency and frequency-converting the resultant frequency into a transmission frequency. If, however, an intermediate frequency on the transmission side is simply set regardless of a frequency on the reception side, since the transmission and reception sides have no correlation, a total of four local oscillators, i.e., local oscillators for obtaining IF signals and local oscillators for performing quadrature modulation and demodulation, are required for the transmission and reception sides, respectively, resulting in complication in the circuit arrangement.